JP6466416B2 - High speed photodetector - Google Patents

Description

  The present invention relates to semiconductor-based photodetectors, and more particularly to a high-speed photodetector structure with improved speed response and a method of manufacturing the same.

  Semiconductor-based photodetectors that can convert an optical signal into a detectable electrical signal are widely used in various technical fields such as optical communication networks. That is, a photodiode based on a pin junction, also called a PIN photodiode, is particularly suitable as a high-speed photodetector because of its quick response to incident light compared to a pn junction. Conventional pin junctions are intrinsic (doped) called p-type semiconductor layers (p layers), n-type semiconductor layers (n layers), and i layers sandwiched between p and n layers. Non- or lightly doped) semiconductor layer. When operating as a photodetector, the pin junction is often in a reverse bias situation and basically does not conduct current without light. When a photon with sufficient energy to excite electrons from the valence band level to the conduction band level is absorbed by the i layer, a pair of “free” electron-hole carriers is generated. Under the influence of a strong potential gradient formed by the lower energy levels provided by the p-layer for holes and by the n-layer for electrons, “free” electrons and hole carriers, respectively, Quickly move in the opposite direction in the i layer towards the layer, thereby generating a photocurrent that can be detected as an electronic signal by an external circuit and associated with the incident optical signal. Conventionally, ohmic contacts are provided on each of the p-layer and n-layer, thereby providing the anode contact and cathode contact of the PIN photodiode, respectively.

  In conventional PIN photodetectors, the photocurrent is essentially due to “free” carriers that occur in the intrinsic layer that functions as a light absorbing layer. Since the same depleted semiconductor layer is used for light absorption and transport of photogenerated carriers between the p-type and n-type semiconductor regions, the bandwidth of the conventional PIN photodetector, ie, incident light The response speed to is often limited by the very slow transport time of hole carriers in the intrinsic layer compared to electron carriers.

  A common way to increase the photodetector bandwidth is to reduce the vertical length (ie, height or thickness) of the light absorbing layer so as to reduce the charge carrier transport time. Since this directly leads to a larger specific capacity, the area of the photodiode and thereby the light detection area must be reduced in order to keep the capacity below a certain specified value. At the same time, the quantum efficiency or responsiveness of the photodiode decreases with decreasing thickness of the light absorbing region.

  Thus, improvements in PIN-based photodetector characteristics suggest a trade-off between bandwidth on the one hand and response and size of the photodetection region on the other hand, and any of these parameters can be Often determined by what is most important for the intended use.

  An approach to reduce the specific capacity is to introduce an intrinsic drift layer that does not absorb light in the wavelength range of interest. Generally speaking, the carriers generated in the light absorption layer travel to the collection layer and the electrode at a drift velocity proportional to the electric field and the respective carrier mobility. In general, the carrier velocity increases with the electric field until it saturates. However, saturation rates for electron carriers are usually achieved at lower electric fields than saturation rates for hole carriers. Thus, as in most conventional systems, the applied external electric field is limited and hole carriers often do not reach their saturation rate. Since the holes travel slower than the electrons, the photodiode is often irradiated from the p side, and it should be understood that the effective distance traveled by the holes is shorter. By designing the photodetector so that only faster carriers (electrons) must travel through the additional drift layer, the drift layer contributes only slightly to the overall transport time, but the photodiode Reducing the specific capacity has a great effect.

  An approach to improve the frequency response and saturation output of a PIN photodiode has been proposed in patent US 5,818,096. This approach is to separate the functions of light absorption and carrier travel between the two semiconductor layers instead of using the same depleted intrinsic semiconductor layer as in a conventional PIN photodiode. In particular, the p-type semiconductor layer is used as a light absorption layer, and the intrinsic non-absorption semiconductor layer is used as a carrier transport layer. In this configuration, the response time of carrier injection into the carrier transit layer is basically determined by the electron diffusion time in the p-type light absorption layer. Since hole carriers only respond with respect to the movement of electrons in the light-absorbing layer, the response time of hole carriers in the light-absorbing layer is very short, so the slower drift velocity of hole carriers in the electron-transporting layer is the response of the photodiode. Does not directly contribute to As a result, the frequency response and saturation output are improved. However, this increase in saturation output suggests that the response of the photodiode is reduced.

  Published patent application US2007 / 0096240A1 proposes a photodiode structure which improves the response in exchange for a lower saturation output. The proposed photodiode structure includes a p-doped layer and an n-doped layer as a further light absorbing layer in addition to a general intrinsic light absorbing layer. In this case, the movement of minority carriers in the doped (non-depleted) absorption layer (i.e. carriers of the opposite polarity to the doping carriers) is basically determined by the respective diffusion times. Furthermore, minority carriers then diffuse rapidly from the doped absorption layer to the intrinsic layer, and therefore do not significantly affect the total transport time compared to conventional PIN photodiodes. The further doped absorption layer increases the overall light absorption volume, so that the photodiode response is also increased.

  Patent application WO 03/0665416 describes an improved PIN photodiode to increase device responsiveness without greatly reducing bandwidth. The proposed photodiode has a p-type semiconductor layer and an n-type semiconductor layer coupled by a second p-type semiconductor layer that functions as a light absorption layer. The second p-type semiconductor layer has a graded p-doping concentration that varies from a high value near the anode to a low value toward the cathode along the carrier path. The graded p-doping concentration increases the net absorption of the photodiode without significantly reducing the transport time of carriers in the absorption layer. Such graded doping increases capacity compared to intrinsic semiconductors of the same thickness, but the pseudo electric field generated by graded doping may give electrons a faster rate to compensate for the increased capacity. There is.

  Accordingly, there is a need for a high speed photodetector that can provide improved response time while maintaining a desirable balance between photodetector responsivity and capacity.

  The present invention has been made in view of the above-mentioned drawbacks and disadvantages of existing systems, the purpose of which is to improve response speed while maintaining the quantum efficiency and specific capacity of the photodetector within desired levels. To provide a photodetector with

  This object is solved by the subject matter of the independent claims. Useful embodiments are defined by the subject matter of the dependent claims.

In accordance with the present invention, a first layer comprising a first semiconductor material having a first bandgap energy configured to absorb light of a wavelength within an intended range, the photodetector; A second layer connected to an adjacent side of the first layer, the second layer including a second semiconductor material having a second band gap energy higher than the first band gap energy; And a doping concentration distribution in at least one of the first layer, the second layer, and a region between the first and second layers is established in the second layer. A photodetector is provided in which the non-zero electric field is less than the electric field established in the first layer under the same reverse bias conditions.

  In a further development of the invention, the doping concentration is such that at least part of the first layer is substantially depleted under the reverse bias situation.

  According to a further development of the invention, the doping concentration in the second layer is higher than the doping concentration in the first layer.

According to a further development of the invention, the doping concentration in the region of the first layer adjacent to the second layer is in the majority of the first layer outside the region of the first layer. Higher than the doping concentration.

  In another development of the invention, the doping concentration in the region of the second layer adjacent to the first layer is such that the majority of the second layer outside the region of the second layer. Higher than doping concentration.

  In a further development of the invention, the second semiconductor material may be a lightly doped n-type semiconductor material.

  In a further development of the invention, the first band gap energy and the second band gap energy are substantially uniform along the thickness of the first layer and the second layer.

  In a further development of the invention, the photodetector is further a third layer connecting the first layer to the second layer, wherein the band is inclined along the thickness of the third layer. The third layer comprising a third semiconductor material having gap energy is provided.

  In a further development of the invention, the tilted bandgap energy is from a value substantially equal to the first bandgap energy on the side of the third layer facing the first layer, the second bandgap energy. Increasing to a value substantially equal to the second bandgap energy on the side of the third layer facing the layer.

  According to a further development of the invention, the third semiconductor material has a composition substantially equal to the first semiconductor material on the side of the third layer facing the first layer, It may have a graded composition that gradually changes to a composition substantially equal to the second semiconductor material on the side of the third layer facing the second layer. In addition or alternatively, the third semiconductor material may include a region doped with a dopant of the same type as the dopant of the second layer.

  In a further development of the invention, the third layer is included in the second layer and / or the first layer, and the third layer is comprised of the second layer and / or the first layer. It is provided on the end side of the layer.

  According to a further development of the present invention, the photodetector further includes a first ohmic contact configured to be coupled to an external electrical circuit, the first ohmic contact, and the second layer. Is a fourth layer disposed between the opposing side of the first layer on the opposite side and comprising a fourth semiconductor material having a fourth band gap energy and a fourth doping concentration. The fourth layer is configured to function as a current diffusion layer for facilitating extraction of a transport current from the first layer to the first ohmic contact.

  The fourth band gap energy may be higher than the first band gap energy so that the fourth layer is a window layer.

  In yet another development of the invention, the photodetector further comprises a second ohmic contact configured to be coupled to an external electrical circuit, the second ohmic contact, and the first layer. Is a fifth layer disposed between the opposing side of the second layer on the opposite side and comprising a fifth semiconductor material having a fifth band gap energy and a fifth doping concentration The fifth layer is configured to function as a current diffusion layer for facilitating extraction of a transport current from the second layer to the second ohmic contact.

  In a further development, at least one of the first and second ohmic electrodes has a ring shape.

  According to a further development of the present invention, the photodetector is further a seventh layer disposed between the second layer and the fifth layer, wherein the seventh band gap energy and A seventh layer comprising a seventh semiconductor material having a seventh doping concentration, wherein the seventh band gap energy is such that minority carrier flow through the seventh layer is interrupted. Higher than the fifth bandgap energy and the fifth bandgap energy.

  According to a further development of the invention, the first semiconductor material comprises a GaAs compound, and at least one of the second to seventh semiconductor materials comprises an AlGaAs compound.

  The accompanying drawings are incorporated in and form a part of this specification to illustrate the principles of the invention. The drawings should not be construed to limit the invention to only the illustrated and described examples of how the invention can be made and used. The drawings are only for the purpose of illustrating advantageous or alternative examples of how the invention can be made and used, and are to be construed as limiting the invention only to the illustrated embodiments. Should not. Furthermore, some aspects of this embodiment may form a solution to the problem of the present invention individually or in various combinations. The embodiments described below can therefore be considered either alone or in any combination thereof.

Further features and advantages will become apparent from the following more specific description of various embodiments of the invention illustrated in the accompanying drawings, wherein like reference numerals refer to like elements.
1 shows a cross-sectional view of a photodetector according to an embodiment of the present invention. The band diagram of the photodetector shown in FIG. 1 is shown.

  Advantageous embodiments of photodetectors constructed in accordance with the invention will be described in more detail with reference to the accompanying drawings.

  As described below, the term “intrinsic semiconductor” should be understood within its general sense of the art as pure, undoped, and unintentionally doped lightly doped. It is also possible to include a modified semiconductor. The term “doped” is used to refer to an n-type or p-type doped semiconductor having a doping concentration higher than that of the intrinsic semiconductor described below.

  FIG. 1 shows a cross-sectional view of a photodetector according to an embodiment of the present invention. In the illustrated embodiment, the photodetector 100 is irradiated vertically from the upper side, which is the side of the photodetector 100 opposite to the substrate 101. However, other configurations in which the incident light illuminates the bottom side, i.e. the side of the photodetector substrate 101, may be envisaged. Furthermore, the terms “top” and “bottom” should be construed as relative terms used only to identify the opposite sides of the photodetector 100, and the photodetector in use. It should not be construed as limiting with respect to 100 physical orientations.

  The photodetector 100 is disposed on the adjacent side of the first semiconductor layer 102 functioning as the light absorption layer and the light absorption layer 102 opposite to the incident light 106 side, and is connected to the light absorption layer 102. A semiconductor layer 104. As will be described later, the second semiconductor layer 104 functions as a carrier traveling layer also called a drift layer. The photodetector 100 further includes a third semiconductor layer 105 disposed between the light absorption layer 102 and the drift layer 104, and the third semiconductor layer 105 is adjacent to the light absorption layer 102 and the drift layer 104. The sides are connected. As will be described later, the third semiconductor layer 105 is an inclined band gap energy layer, that is, a simple inclined layer.

  On the adjacent side of the light absorption layer 102 opposite to the side adjacent to the drift layer 104, the photodiode 100 further includes a fourth semiconductor layer 110 made of a doped semiconductor material, and the fourth semiconductor layer 110 includes As described later, it functions as a current diffusion layer. A first contact layer 112 comprising a doped semiconductor material of the same type as the fourth semiconductor layer 110, in the illustrated embodiment a p-type semiconductor, is provided on top of the current spreading layer 110. The first contact layer 112 is for improving the electrical contact of the current spreading layer 110 with the first ohmic contact 114 deposited on top of the photodetector structure. The first contact layer 112 may function as a capping layer. The first ohmic contact 114 formed on the contact layer 112 preferably does not completely cover the underlying semiconductor layer so that the normal incident light 106 can directly illuminate the p-type contact layer 112. It has a ring shape. With this design of the first ohmic contact 114, the incident light 106 is directly transmitted to the lower semiconductor layer without passing through the ohmic contact 114. In other embodiments, the first ohmic contact 114 may comprise other designs, such as a closed circle shape.

  The photodetector 100 further includes a fifth semiconductor layer 116 disposed on the drift layer 104 side opposite to the side adjacent to the light absorption layer 102. Since the fifth semiconductor layer 116 also functions as a current diffusion layer on this side of the photodetector 100, the fifth semiconductor layer 116 has impurities that provide carriers of the opposite polarity to the fourth semiconductor layer 110. A doped semiconductor material is provided. Between the fifth semiconductor layer 116 and the substrate 101, a second contact layer 118 containing the same doping type semiconductor material as the fifth semiconductor layer 116 is provided. As shown in FIG. 1, the second contact layer 118 extends on the substrate 101 beyond the range of vertically stacked semiconductor layers formed above. The second contact layer 118 improves electrical coupling between the current spreading layer 116 and the second ohmic contact 120 deposited on the second contact layer 118. Similar to the first ohmic contact 114, the second ohmic contact 120 is preferably a flat ring deposited on the second contact layer 118 and concentrically disposed around the vertical stack of semiconductor layers. With the design. In other embodiments, the second ohmic contact 120 may employ other designs, for example, two parallel striped electrodes deposited on the second contact layer 118 on each side of the vertical stack. May be provided.

  The first ohmic contact 114 and the second ohmic contact 120 are preferably made of a conductive material such as aluminum, silver, gold, or copper. In the present embodiment, the first and second ohmic contacts function as an anode contact and a cathode contact for electrically connecting the photodiode 100 to an external circuit (not shown), respectively.

  As will be described with reference to FIG. 1, the photodetector 100 is formed as a heterostructure of layers stacked vertically on a substrate 101. In the present embodiment, the substrate is made of a GaAs compound. However, other types of substrates may be used. That is, the substrate may be of a material suitable for providing a distributed Bragg reflector (DBR) such as an AlGaAs compound. In other embodiments, the second contact layer 118 itself may function as a substrate for the photodetector structure.

Further details of the layers forming the photodetector 100 will be described with reference to FIG. 2, which shows a band diagram for the photodetector 100 shown in FIG. The band diagram 200 schematically shows the change of the conduction band edge 202 and the valence band edge 204 along the thickness of the photodetector layer in the vertical direction 124 shown in FIG. The band gap energy for each layer is depicted as the separation between the conduction band edge 202 and the valence band edge 204 of each layer in the vertical direction of the band diagram 200. The horizontal direction in the band diagram 200 represents a conductive path of carriers between the anode 114 and the cathode 120 of the photodetector 100 shown in FIG.

  The principle underlying the structure of the photodetector 100 is to use a thin light-absorbing layer with a high applied electric field combined with a drift layer specified by high mobility at a much lower electric field due to the trimmed doping profile. It is in.

It is advantageous to design the photodetector 100 such that only charge carriers with higher mobility must travel through the drift layer 104. This principle is demonstrated by one embodiment that is particularly advantageous for semiconductor materials in which electrons have a higher mobility than holes, such as GaAs and Al _x Ga _1-x As (X = 0... 1) compounds. ing. In this embodiment, only electrons (carriers with higher mobility) must travel through the drift layer 104, but holes (carriers with lower mobility) must reach the current spreading layer 110 before reaching the current spreading layer 110. It is only necessary to travel a distance less than or at most equal to the thickness of the light absorption layer 102.

  In the photodetector 100, the absorption layer 102 comprises an intrinsic semiconductor material and allows the generation of electron-hole pairs in the light absorption layer 102 by absorption of photons having wavelengths in the range of interest. It has energy 206. In the specific case where the target range is close to a wavelength of 850 nm, the light absorbing layer 102 is preferably made of intrinsic GaAs (i-GaAs) semiconductor material. The lower band gap energy of undoped GaAs relative to other intrinsic semiconductor materials is suitable for light absorption at wavelengths of interest in optical communications. The vertical length (thickness) of the light absorbing layer 102 may be customized according to the specific characteristics desired for the photodetector 100. A range suitable for the thickness of the light absorption layer 102 is about 0.1 μm to 2 μm. Due to its undoped state, the light absorption layer 102 is basically depleted during operation of the photodetector 100 and a high electric field profile (potential gradient) is present in the light absorption layer 102 as required. Helps to be established. The high electric field makes it possible to reduce the transport time of “free” carriers in the light absorbing layer 102.

The drift layer 104 comprises a semiconductor material having a band gap energy 208 such that the drift layer 104 basically functions as a carrier transport layer. As shown in FIG. 2, the band gap energy 208 of the drift layer 104 is larger than the band gap energy 206 of the light absorption layer 102. In particular, the bandgap energy 208 is large enough to prevent or at least significantly reduce light absorption by the drift layer 104 at light wavelengths within the range of interest. In the illustrated embodiment, the drift layer 104 includes an intrinsic or lightly n-type doped AlGaAs (Al _x Ga _1-x As, X = 0... 1) compound semiconductor alloy. Preferably, a vertical thickness of up to 5 μm may be used for the drift layer 104. The particular thickness used depends on the relative thickness of the other layers, in particular the light absorbing layer 102, and may be customized for the photodetector 100 according to the desired characteristics.

  The electric field in the drift layer 104 is lower than the electric field in the light absorption layer 102 as will be described later. The reduction in the electric field in the drift layer 104 relative to the light absorbing layer 102 is depicted in FIG. 2 as a change from a higher slope at the valence band edge 204 and conduction band edge 202 for the light absorbing layer 102 to a lower slope for the drift layer 104. ing. This unique gradient profile is in contrast to the conventional constant gradient that appears throughout the absorption and collection layer structures in known PIN photodiode structures such as the background art described above. Despite the difference in gradient between the valence band edge and the conduction band edge between the light absorption layer 102 and the drift layer 104 as depicted in FIG. 2, the band gap energy of the light absorption layer 102 and the drift layer 104 is It remains essentially uniform along the thickness of each layer.

  In this embodiment, the transition from the light absorption layer 102 with a higher electric field to the drift layer 104 with a lower electric field is performed by the gradient layer 105. The graded layer 105 comprises a semiconductor material having a composition that gradually changes along its vertical thickness. Such a step change in the semiconductor composition is achieved by a corresponding step change in the band gap energy. Preferably, the semiconductor composition in the graded layer 105 is such that the band gap energy is substantially equal to the band gap energy 206 of the light absorbing layer 102 on the side of the graded layer 105 immediately adjacent to the light absorbing layer 102 and the drift layer 104. On the side of the graded layer 105 immediately adjacent to the gradient layer 105 so as to gradually change along the vertical thickness of the graded layer 105 toward a second bandgap energy substantially equal to the bandgap energy 208 of the drift layer 104. . In the present embodiment, the composition of the semiconductor material changes so as to obtain a stepwise increase in band gap energy along the inclined layer 105 toward the drift layer 104 shown in FIG.

  The reduction of the electric field in the drift layer 104 relative to the light absorption layer 102 is due to the distribution of the dopant concentration in the light absorption layer 102, the drift layer 104, and the region between the two layers, including the graded layer 105 in the illustrated embodiment. Realized. In an advantageous embodiment, the electric field in the drift layer 104 is reduced by n-type doping the region of the graded layer 105 adjacent to the light absorbing layer 102. This doped region may be substantially thinner than the graded layer 105, but may be the same thickness as the graded layer 105. To achieve the desired reduction in the electric field in the drift layer 104, a thinner doping region requires a higher doping level, but a thicker region requires a lower doping level. It is recognized that the main factor that achieves a specific reduction in the electric field in the drift layer 104 is the total fixed specific charge due to the n-type dopant introduced into the doped region directly adjacent to the light absorbing layer 102. In other embodiments of the present invention, the reduction of the electric field in the drift layer 104 is achieved by the n-type doping of most of the drift layer 104 itself. Also, in this embodiment, it is recognized that the electric field decrease in the drift layer 104 is controlled by the total fixed charge by the n-type dopant per unit area. Other embodiments may combine some doping of the graded layer 105 and some or all of the drift layer 104. In any case, the concentration of the n-type dopant that causes the desired reduction in the electric field within the drift layer 104 is the layer that participates in carrier transport but does not fundamentally absorb light in the region of interest, ie, the graded layer 105 and the drift layer. Distributed in layer 104. Still other embodiments comprise a significantly higher doping level placed at a particular location in the drift layer 104 or the region comprising the layers 105 and 104 and / or the region of the layer 102 adjacent to the layer 104 1. Variable doping levels distributed along one or several very narrow areas may be used. The dopant concentration distribution described above may be achieved using doping techniques that are not described herein as they are well known in the art.

  It is also generally observed that charge carriers have higher mobility in intrinsic semiconductor materials and that this mobility decreases with increasing doping concentration in this material. Thus, it may be advantageous to employ an intrinsically or lightly doped drift layer 104, but the reduction of the electric field in the drift layer 104 is primarily due to reduced mobility (caused by doping) with respect to carrier transport time. This is achieved by doping only a small part of the graded layer 105 so that the effect of.

In the illustrated embodiment, where the light absorbing layer 102 comprises an intrinsic GaAs semiconductor material and the drift layer 104 comprises an intrinsic or lightly n-doped AlGaAs compound, the graded layer 105 is lightly n-doped comprising a graded concentration of aluminum. The prepared Al _x Ga _1-x As compound. At this time, the parameter x employs a substantially zero value (GaAs) on the side directly adjacent to the light absorption layer 102, and Al _x Ga used for the drift layer 104 on the side directly adjacent to the drift layer 104. It gradually increases across the gradient layer 105 until a value similar to the x concentration of _1-x As is reached. The graded bandgap energy basically generates a pseudo-electric field in the graded layer 105 that acts on minority carriers, ie, holes in the case of n-doped semiconductor layers. In this embodiment, the graded layer 105 is a slightly doped n-type semiconductor. Therefore, the generated pseudo electric field basically acts on hole carriers injected from the drift layer 104 by increasing the hole transport rate. This effect is illustrated in FIG. 2 by the increased gradient of the valence band edge of the graded layer 105. Further, the gradient layer 105 compensates for a mismatch between the light absorption layer 102 and the drift layer 104 due to the difference in band gap energy between the light absorption layer 102 and the drift layer 104. Furthermore, since the band gap energy of the gradient layer 105 in the region near the light absorption layer 102 is equal to the band gap energy of the light absorption layer 102, incident photons are absorbed above a part of the gradient layer 105. There is a possibility. In other embodiments, the graded layer 105 may exhibit a graded doping concentration in addition to or instead of the graded semiconductor compound. The graded doping concentration creates an additional electric field acting on both types of carriers, i.e. holes and electrons, and thus contributes to reducing the respective transport time. Therefore, by carefully adjusting the electric field that varies across the pin bond, shorter transport times for both types of carriers can be achieved.

In other embodiments, the graded layer 105 may be implemented as a multilayer structure of two or more layers of Al _x Ga _1-x As compounds, each layer having a concentration x in the preceding layer (s). Has a gradually increasing concentration x. In other embodiments, the band gap gradient effect providing graded layer 105 may be replaced by a region of graded composition incorporated into either or both of the light absorbing layer 102 and the drift layer 104. In this configuration, the graded layer 105 is not physically present, but is included in the light absorbing layer 102 and / or the drift layer 104, where the light absorbing layer 102 and the drift layer 104 are in this case their respective adjacent neighbors. Connected directly by the side. The gradient in the material composition similar to that of the gradient layer 105 described above is that the material composition of most of the light absorption layer 102 (or drift layer 104) and the material composition of most of the drift layer 104 (or light absorption layer 102) from doping. Or in the end region of the light absorption layer 102 connected to the drift layer 104 or in the end region of the drift layer 104 connected to the light absorption layer 102 so that a gradual transition to doping is achieved. .

  As described above, the p-side current diffusion layer 110 is disposed between the light absorption layer 102 and the p contact layer 112. The p-side current diffusion layer 110 has a function of supporting the extraction of the current of the photodetector 100 to the anode 114. That is, when the anode 114 is a ring contact, the p-side current diffusion layer 110 assists in reducing the resistance of the photodetector 100. Preferably, the p-side current diffusion layer 110 includes a semiconductor material having a band gap energy sufficiently higher than the band gap energy 206 of the light absorption layer 102 so as to limit light absorption to the light absorption layer 102 directly adjacent thereto. In this sense, the p-side current spreading layer 110 is used as a window layer to allow light incident on the upper side of the photodetector 100 (that is, the p-side in the illustrated embodiment) to pass without being attenuated. Function. Furthermore, the extremely high band gap energy of the p-side current diffusion layer 110 compared to the light absorption layer 102 also prevents the electron carriers generated in the light absorption layer 102 from flowing toward the anode 114. The p-side current diffusion layer 110 also contributes to blocking the flow of electrons from the anode 114 that adds a noise component to the optical response of the photodetector 100 to the light absorption layer 102. Preferably, the semiconductor material used for the p-side current spreading layer 110 is a p-doped AlGaAs compound. The doping concentration of the p-side current diffusion layer 110 is preferably higher than the doping concentration of the light absorption layer 102 and significantly lower than the doping concentration of the p contact layer 112. Preferably, the p contact layer 112 is a heavily doped p-type semiconductor such as a p-doped GaAs compound.

  As described above, the photodetector 100 includes the current diffusion layer 116 on the n side of the photodetector 100 as well. The n-side current spreading layer 116 provides a function similar to that of the p-side current spreading layer 110, but is adapted to the characteristics of the n-doped side of the photodetector 100. In particular, the n-side current spreading layer 116 preferably does not contribute to the absorption of incident light at the wavelength of interest, and drift layer 104, graded layer 105 so as to promote the movement of photogenerated hole carriers towards the anode 114. And an n-doped semiconductor material having a band gap energy higher than the band gap energy 208 of the light absorption layer 102. Furthermore, the band gap energy of the n-side current diffusion layer 116 is significantly higher than the band gap energy of the n contact layer 118. Accordingly, the flow of hole carriers from the n-contact layer 118 to the drift layer 104 is also blocked, thereby reducing the noise component in the response of the photodetector 100. Preferably, the n-side current diffusion layer 116 is provided as an n-doped AlGaAs compound layer. A suitable material for the n-contact layer 118 is an n-doped GaAs material.

  The thickness of the current spreading layer is customized according to the characteristics of the photo detector 100, such as the thickness of other layers of the photo detector structure, and their specific doping concentration is used to provide the desired current spreading function. Any of those commonly used in the art may be used. Preferably, the thickness of the p-side current spreading layer 110 and / or the n-side current spreading layer 116 is substantially between 1 μm and 2 μm. In other embodiments, the presence of a current spreading layer on the p-side and / or n-side of the photodetector may be omitted. For example, in an embodiment where the photodetector has an n contact layer that also functions as a substrate, the function of the current spreading layer on the n side of the photodetector is less important because the entire bottom of the photodetector is conductive. Absent. In such a configuration, the n-side current diffusion layer may be omitted.

  A further contribution to block hole carrier flow from the cathode side to the drift layer 104 and the light absorption layer 102 is provided by a thin semiconductor barrier layer 122 disposed between the drift layer 104 and the n-side current spreading layer 116. May be. If hole carriers from the cathode side enter the drift layer 104, the hole carriers are swept out into the drift and absorption region toward the anode 114, creating an additional noise component in the photodetector response. If such noise components are not important, the barrier layer 122 may be omitted in the structure of the photodetector 100. The doping concentration and bandgap energy for the barrier layer 122 are selected to provide a desired carrier shielding effect against the flow of minority carriers in the barrier layer 122 itself, i.e., the opposite polarity to the polarity of the doping. . Preferably, the band gap energy of the barrier layer 122 is larger than the band gap energy of the adjacent drift layer 104 and the band gap energy of the n-side current diffusion layer 116. In the illustrated embodiment, the barrier layer 122 comprises an n-doped AlGaAs compound having a vertical thickness of approximately 20 nm. However, other semiconductor materials and / or doping concentrations can be readily determined by those skilled in the art based on specific parameters of the photodetector to achieve the desired blocking effect for the barrier layer. In other embodiments, the barrier layer 122 may also function as an etch stop layer.

  In other embodiments, the photodetector 100 is further disposed between the light absorbing layer 102 and the p-side current spreading layer 110 to block the flow of electron carriers from the anode 114 to the light absorbing layer 102. A p-doped semiconductor barrier layer may be provided. The p-doped and n-doped barrier layers may then be provided on both or only the p-side and n-side of the photodetector, depending on the specific application.

Exemplary values for doping levels and thicknesses used to form the GaAs / AlGaAs-based structure of photodetector 100 as described above are summarized in Table 1. As commonly used in the art, the symbol p ⁺⁺ refers to heavily p-doped material, and p ⁺ and n ⁺ refer to moderate levels of p-doping and n-doping, respectively.

  Although not shown in FIGS. 1 and 2, the photodetector 100 may include additional functional layers to improve performance. For example, a less doped “setback” layer may be inserted next to the intrinsic semiconductor layer. Further, while the above embodiments have been described with reference to photodetectors having a heterostructure of layers based on GaAs and AlGaAs materials, the present invention is directed to the selected semiconductor material having the above described band gap energy and / or Other compounds or basic semiconductor materials may be used instead of GaAa and AlGaAs as long as they have band gap energy and / or doping concentration according to the relative relationship of doping concentration. Furthermore, although the invention has been described with reference to a vertically illuminated structure, the principles of the invention may be applied to photodetectors having a waveguide structure. As will be readily appreciated by those skilled in the art, the terms “vertically illuminated” and “vertically stacked structure” are not intended to limit the use or structure of the photodetector to a vertical orientation, Other orientations may be employed, such as detection of optical signals along the structure and / or photodetectors having a vertically stacked structure. Furthermore, the principle of the photodetector structure described above is advantageous because it can be embedded in a resonant cavity enhanced (RCE) type photodetector.

DESCRIPTION OF SYMBOLS 100 Photodetector 101 Substrate 102 Light absorption layer 104 Drift layer 106 Incident light 105 Inclined band gap layer (gradient layer)
110 window, p-type current spreading layer 112 capping and p contact layer 114 anode 116 n side current spreading layer 118 n contact layer 120 cathode 122 barrier to minority carriers (holes) (and can serve as an etch stop)
124 vertical direction 200 band gap diagram 202 conduction band edge 204 valence band edge 206 band gap energy 208 of light absorption layer 208 band gap energy of drift layer

Claims (14)

A photodetector,
A first band gap energy that is configured to absorb light of a wavelength within the intended scope, a first field, possess a first doping concentration, a first semiconductor material comprising a GaAs compound A first layer comprising:
A second layer connected to an adjacent side of the first layer and having a second band gap energy higher than the first band gap energy and comprising a second semiconductor material comprising an AlGaAs compound ;
A third layer connecting the first layer to the second layer, the third layer comprising a third semiconductor material having a graded bandgap energy along the thickness of the third layer; ,
A fourth layer disposed on an adjacent side of the first layer opposite to the second layer and comprising a fourth semiconductor material having a fourth band gap energy and a fourth doping concentration;
The doping concentration distribution in the first layer, the second layer, and the third layer indicates that the non-zero electric field established in the second layer is the same under the same reverse bias condition. Less than the electric field established in the layer of
The tilted bandgap energy is from a value substantially equal to the first bandgap energy on the third layer side facing the first layer, from the third layer facing the second layer. Increasing to a value substantially equal to the second band gap energy on the layer side;
The photodetector further includes a contact layer disposed on an adjacent side of the fourth layer opposite to the first layer, the contact layer being greater than a bandgap energy and a fourth doping concentration. A photodetector comprising a semiconductor material having a doping concentration, wherein incident light is directly incident on the contact layer . Under the reverse bias condition , at least a portion of the first layer is substantially depleted and the doping concentration in the second layer is higher than the doping concentration in the first layer. Item 2. The photodetector according to Item 1 . The photodetector according to claim 1 , wherein the second semiconductor material is a lightly doped semiconductor material. The first band gap energy and the second band gap energy are substantially uniform along the thickness of the first layer and the second layer,
The photodetector according to claim 1 . The third semiconductor material faces the second layer from a composition substantially equal to the first semiconductor material on the side of the third layer facing the first layer. the photodetector of claim 1 having a graded composition varying gradually in the second semiconductor material substantially equal composition on the side of the layer. The photodetector of claim 1 , wherein the third semiconductor material comprises a region doped with a dopant of the same type as the dopant of the second layer. A first ohmic contact coupled to the external electrical circuit;
The light detection according to claim 1 , wherein the fourth layer is configured to function as a current diffusion layer for facilitating extraction of a transport current from the first layer to the first ohmic contact. vessel. The photodetector according to claim 1 , wherein the first layer has the first doping concentration, and the fourth doping concentration is higher than the first doping concentration. The photodetector according to claim 1 , wherein the fourth band gap energy is higher than the first band gap energy. Disposed between a second ohmic contact configured to be coupled to an external electrical circuit, and a side surface of the second layer opposite the first ohmic contact and the first layer A fifth layer,
The photodetector according to claim 1 , wherein the fifth layer has a fifth band gap energy higher than the second band gap energy and has a fifth doping concentration. The photodetector according to claim 10 , wherein at least one of the first and second ohmic contacts has a ring shape. Further comprising a barrier layer disposed between the second layer and the fifth layer;
The barrier layer includes a sixth semiconductor material having a sixth bandgap energy and a sixth doping concentration;
The photodetector according to claim 10 , wherein the sixth band gap energy is higher than the second band gap energy and the fifth band gap energy. The sixth semiconductor material comprises an n-doped AlGaAs compound;
The photodetector according to claim 12 , wherein the barrier layer has a thickness of 20 nm. The first semiconductor material comprises an intrinsic GaAs compound;
The second semiconductor material comprises an intrinsic or lightly n-doped AlGaAs compound;
The third semiconductor material includes a lightly n-doped AlGaAs compound in which the concentration of aluminum changes stepwise;
The aluminum concentration is substantially zero on the first side of the third layer adjacent to the first layer and the second concentration of the third layer adjacent to the second layer . Gradually increasing over the third layer towards the side of
The photodetector according to claim 1, wherein a concentration of the aluminum on the second side is substantially equal to a concentration of aluminum of the AlGaAs compound of the second semiconductor material.

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